Before all you men run out to start buying barnacles, it refers to their own penises.

In the peculiar world of barnacle sex, shape matters just as much as size.

To cope with changing tides and a sedentary lifestyle, the gnarly crustaceans have evolved penises that are eight times the length of their bodies—the longest relative to body size of any animal.

Now new research reveals that the creatures' lengthy members can also change shape to suit their environment.

Barnacles living in gentle waters have long, thin penises best equipped for maximum reach, the study found.

But those animals living in rough waters have shorter, stouter penises that are better able to withstand strong waves.

The findings suggest an animal's environment can sculpt its genitals as thoroughly as female choice or competition between males.

"It's just an example that says, Look, there's other things that can also be important for shaping the evolution of genitalia," said study co-author Christopher Neufeld of the University of Alberta, Canada.

Neufeld and colleague Richard Palmer describe the work online this month in the journal Proceedings of the Royal Society B.

Penis Pump

Despite being hermaphrodites that have both male and female genitalia, barnacles prefer to mate with other individuals whenever possible.

But as one of the few immobile species to use direct insemination, barnacles face some unique challenges.

Their penises have to be long enough to find and inseminate distant neighbors, but not so long that they flop around uselessly in strong waves.

Previous work by Palmer's lab had shown that barnacles can change the shape and size of the feathery leg-like appendages they use to catch food depending on wave conditions.

To find out if barnacles also have shape-shifting penises, Neufeld and Palmer compared the penises of white acorn barnacles collected from rough waters against those taken from in protected harbors.

But barnacle penises are retractable and are not always easy to observe.

"It's hard to get barnacles to extend their penises on demand in the lab," Neufeld said.

So the team artificially inflated the barnacles' genitalia with seawater using a custom-made penis pump built out of tubes and hypodermic syringes.

Surprisingly Pliable

The researchers also transplanted barnacles living in gentle waters to rough waters and vice versa, to make sure the penis variations they observed were a result of the environment and not due to genetic differences.

The results showed that barnacles could indeed change the width and length of their penises to suit the wave conditions of their new homes.

Heather Proctor, a biologist at the University of Alberta who was not involved in the study, said she was astonished by how pliable barnacles penises are.

"That runs rather counter to what we know about other arthropod copulatory apparatuses," Proctor said.

The body size of land arthropods such as insects and spiders might vary a lot within a species, but their genitalia tend to remain more or less constant.